As a means to convey nucleic acid or its analogue to target cells or tissues, there have been made various proposals about both of virus vector and synthetic vector (or non-virus vector) as so-called gene carrier or vector. Synthetic vector is considered to have no grave toxicity as compared with virus vector although synthetic vector is not free of some risk of toxicity or the like as brought into question in drug delivery system which has been studied in conventional medical treatment. Furthermore, in synthetic vector, there is no limitation on the size of nucleic acid to be carried, and, moreover, precise molecular designing is possible. For these reasons, assiduous research and development are being made with regard to synthetic vector. Typical examples of synthetic vector include cationic lipid (e.g., lipofectin) and cationic polymer each of which forms an ion complex with negatively charged DNA. In the former, it has been tried to lower the toxicity against cells by changing the molecular structure of lipid, and also to enhance the efficiency of expression of gene in the introduced cells, and, thus, results have been produced to a certain extent (see, e.g., non-patent document 1. Documents are identified below. The same is applied to other documents.). In vivo, however, desired results have not been obtained.
As for the latter, on the other hand, there have long been studied poly(L-lysine), DEAE-dextran, polyethylenimine (see, e.g., non-patent document 2), chitosan (see, e.g., non-patent document 3), etc. They are, however, still unsatisfactory in toxicity against cells, and also in the efficiency of introduction and expression of gene.
Under the above-mentioned circumstances, the inventors of this invention have found out that the use of block copolymer which is made of cationic polymer (e.g., polylysine) and, connected thereto, water-soluble and low-toxicity polyethylene glycol (PEG) forms polymer micelle which is an autonomously associated polyion complex (PIC) with DNA encapsulated therein. The inventors have thus confirmed that such a polymer micelle lowers toxicity, and shows expression efficiency higher than that of lipofectin which is now being most widely employed for in vitro gene introduction. The inventors have also provided a block copolymer which is composed of a segment chain having a specific amine group at side chain as a cationically charged group and PEG chain, as a block copolymer which may be used as a further improved carrier of gene (see patent document 1: Japanese Patent Application KOKAI Publication No. 2004-352972).
List of Documents:
                Patent document 1: Japanese Patent Application KOKAI Publication No. 2004-352972        Non-patent document 1: C. F. Benett et al., J. Drug Targetting, 5, 149 (1997)        Non-patent document 2: O. Boussif et al., Proc. Natl. Acad. Sci., USA, 92, 7297 (1995)        Non-patent document 3: S. C. Richardson et al., Int. J. Pharm. 178, (1999) 231        